I would tend to say that your question is based on a false assumption: "it has no defined gender" - this situation should never happen in French at all as all nouns have a definite gender.

But yeah, in the case you don't want to name the thing or can't find a noun for it you can use "Mange ça!", or even simply "Mange!" if there's nothing else to eat. I'm deliberatley suggesting "Mange" instead of "Mangez" since I feel there isn't much respect involved at this point, but that would of course depend on the context.

The person will understand Mange le fruit ou Mange la pomme with the same end result.

In french every name has a gender, which is a mandatory syntactic attribute, not a matter of biology. We use it to know to which name relates an adjective (with an efficiency of 75%). As 50% of our vocabulary is masculine and 50% is feminine, you will always find a word with the appropriate gender describing your it.

In facts, if your it has no definite gender, it can only be amour, délice, orgue, gens, oeuvre, alvéole, après-midi, autoroute, enzyme, réglisse,... But even then, they take a gender as soon as they enter in a phrase.

About the distinction between grammatical and natural gender, did you know that according to wikipedia, in old English wif (wife) was neutral and wifmann (woman) was masculine ?